'It's not the same as buying part of a uniform required for a specific job, which you knew prior to working there you'd have to have, given that they were never expected to wear one before.'
Uniforms and dress policies change all the time. This is exactly the same thing. And they're hardly asking them to buy something expensive, a face cover costs £5 not £30 or £100.
When a place I waitressed in changed their dress policy to no skirts for safety reasons (following an incident where one was caught on a chair and pulled it over, knocking a whole table over) I had to go out and buy jeans to wear to work as previously I only owned blue jeans and the 'uniform' from the start had been all black. A couple pairs of black jeans costs me way more than a fiver and I was working for minimum wage. Had I not bought them I would not have been able to continue working there - exactly the same thing!
You also say your issue is with that Starbucks is mandating something and then not providing it but in the same post go on to say how does a face cover provide the same level of protection as proper PPE? So which is it you have a problem with?
Starbucks mandating PPE despite not providing it? In which case it is normal in many jobs to require small pieces of equipment/uniform to be bought by the employee
Or Starbucks not providing medical grade PPE?
The same as every other takeaway, supermarket, and business continuing to operate outside the health industry during this pandemic
Both are irrelevant points and it really just seems like you're looking for something nag to moan about, but this shouldn't be it.